1. Field of the Invention
The present invention relates to an improved process of providing a metallic article such as a ferrous strip with a coating such as a zinc coating and more particularly to a "dipless" metallizing process without dipping the article into a molten metallizing bath and to an apparatus for carrying out said process.
2. Description of the Prior Art
Modern galvanizing procedures, in spite of recent improvements, can still be considered an outdated inheritance of the original hot dip galvanizing in which the article to be coated was submerged, while still cold, in a heated zinc pot, thereby passing through a layer of a flux which floats on the molten zinc bath and cleans the article to be coated of any dirt and moisture.
Before galvanizing takes place, the article had to be heated to about the melting point of the zinc. Such heating, of course, takes some time. During heating a brittle layer of a ferro-zinc alloy was formed on the interface between the article and the zinc layer. Said ferro-zinc alloy layer caused the zinc coating to readily flake and peel off the article, thus diminishing considerably the anti-corrosive properties of the galvanized article.
In spite of vast improvements as they are achieved by recent modifications of the known processes, the sheet material, usually in the form of a continuous strip, is still passed through a molten zinc bath after it has been preheated and under the protection of a non-oxidizing atmosphere. Thus, it is no longer necessary to keep the strip in the molten zinc in order to heat it. However, on account of merely geometrical considerations, the strip must remain in contact with the molten zinc for a longer period of time than required for purely metallurgical consideration. As a result of such a prolonged contact of strip and zinc a brittle ferro-zinc alloy of greater thickness than desired is formed. Formation of the ferro-zinc alloy is prevented, at least partly, by the addition of aluminum or the like to the zinc bath. Such addition, however, reduces to some extent the ductility of the zinc coating in comparison to the ductility of a non-alloyed zinc coating.
It follows that the duration of contact between the article and the zinc bath is determined by the use of a zinc bath provided with a sinking drum as well as by the dimensions of such a drum.